The invention relates to a control and data transmission system, in particular for use in a motor vehicle, as claimed in the preamble of claim 1.
At present, numerous bus systems are known in which a multiplicity of subscribers are arranged in series in a serial bus. The data is transmitted from a main device, which is referred to as the bus master, via the bus to all the subscribers which are connected, and then back again to the bus master. The bus subscribers include, for example, sensors and actuators which can transmit and receive data and control signals via the bus. In such bus systems, the data is transmitted either by a voltage modulation or by means of an inductively coupled current modulation of the carrier signal, it being possible with the latter to achieve better EMC characteristics of the bus system in comparison to voltage-modulated data transmission if, for example, a twisted line pair is used as bus. Such a bus system is known, for example, from DE-PS 44 12 921.
The invention is based on the object of providing a control and data transmission system with a serial bus in which the data and control signals to be transmitted are current-modulated and whose components permit simple and flexible installation.
The invention solves this technical problem with the features of claim 1.
To do this, the control and data transmission system has, in addition to a bus which comprises at least one working current loop, a plurality of subscribers and a D.C. voltage source, a controllable bus terminating device which impresses a predetermined, essentially constant working current into the loop-shaped, serial bus. Therefore, the controllable bus terminating device serves to maintain an essentially constant working current in the bus during the operating state of the control and data transmission system in that it can compensate, for example, temperature-induced and aging-induced current fluctuations of the subscribers connected into the bus. The subscribers which can be connected into the bus each have a D.C.-coupled transmitter for generating and emitting control signals and data signals which modulate the working current, and a D.C.-coupled receiver for sensing and evaluating the working current which is modulated by the control signals and data signals. Any desired transmission protocols and data formats can be used to transmit the data and control signals. For example, a specific subscriber can selectively transmit a message to a selected subscriber by virtue of the fact that the data block to be transmitted contains both the destination address and the source address. The message can then be read only by the destination subscriber whose destination address is contained in the data block.
Advantageous developments are given in the subclaims.
In order to increase the flexibility of the control and data transmission system and to facilitate its installation, each transmitter has at least one controlled power source, and the receiver has an ammeter. In addition, the receiver ensures that if there is a potential difference between the input and output of a corresponding subscriber, said difference is kept essentially constant. At this point it is to be noted that the potential difference is expediently the same in all the connected subscribers; however, this is not necessary. Each subscriber is also designed in such a way that it can be switched between transmit mode and receive mode. The switching over between transmit mode and receive mode is carried out, for example, by connecting the transmitter into the bus in the transmit mode, and the receiver into the bus in the receive mode.
Because the working current which is to be modulated flows through all the subscribers, data transmission can take place both in the forward direction and in the reverse direction via the bus given an appropriate dimensioning of the controllable bus terminating device.
The flexibility of the control and data transmission system can also be increased by virtue of the fact that at least one subscriber is capable of serving as a master, that is to say can transmit not only user data but also control data to the other subscribers.
Installation of the control and data transmission system can be simplified by virtue of the fact that each subscriber has a D.C. voltage supply source and/or a controllable bus terminating device. In this way, it is possible to avoid the use of a separate D.C. voltage source and/or of a separate controllable bus terminating device. Each subscriber instead contains all the components themselves which are necessary for forming a functional control and data transmission system. The subscribers can in addition be manufactured as cost-effective, standardized components by virtue of the fact that they are each composed of at least one, preferably identical integrated circuit. With such implemented subscribers it is possible to disconnect the working current loop of the bus downstream of any of the subscribers without the bus losing its structure, and thus its functional capability up to the point of interruption. This capability is achieved by means of the controllable bus terminating device which is integrated in each subscriber. If the bus is a two-wire line, the bus terminating device of the subscriber which is located in front of the interruption point closes the working current loop in the bus.
The bus expediently comprises a two-wire line in the form of a ring line or spur line.
So that it is ensured that the controllable bus terminating device compensates only relatively slow interference fluctuations in comparison to the data transmission rate, a sufficiently long time constant in comparison to the data transmission rate or bit length is to be selected.
In order to avoid additional lines for supplying power to the individual subscribers, power is also supplied via the bus.
The efficiency of the control and data transmission system is increased by virtue of the fact that the system can configure itself after being put into service or after an interruption, in particular of the bus.